1. Field of the Invention
This invention relates to a spark plug with a built-in pressure sensor, which is to be fitted in an internal combustion engine. In particular, this invention is concerned with a watertight structure for the accommodation of a pressure sensor disposed in a base portion of a metal shell of the spark plug.
2. Description of the Related Art
Coupled with the recent move toward internal combustion engines of higher performance, there is an outstanding demand for the maintenance of accurate ignition timing during high-speed and high-compression operation. With a view to achieving accurate ignition timing for each combustion chamber of an internal combustion engine and hence ideal combustion there, it has been proposed to provide a spark plug, which is to be fitted in the combustion chamber, with a built-in pressure sensor so that detection of knocking or the like inside the combustion engine can be utilized to control the ignition timing [Japanese Utility Model Publication (Kokoku) No. SHO 60-30790 published on Aug. 7, 1987]. In the spark plug with the built-in pressure sensor, each signal which has been generated by a piezoelectric element of the pressure sensor is outputted via an electrode plate or the like upon transmission of the signal to a central processing unit (CPU) to control the ignition timing or the like. The pressure sensor itself is therefore required to have a high degree of watertightness. To attain such a high degree of watertightness, some spark plugs have been proposed accordingly, including a spark plug in which a pressure sensor is accommodated in a metal casing and the metal casing with the pressure sensor built therein is joined and fixed on a base portion of a metal shell by seam welding [Japanese Utility Model Application Laid-Open (Kokai) No. SHO 63-171988 laid open on Nov. 9, 1988] and a spark plug in which a pressure sensor is sealed together with a molding material such as a resin within a metal casing and the metal casing is fixed via a flanged ring joined and fixed on a metal shell by seam welding [Japanese Patent Publication (Kokoku) No. HEI 2-54637 published on Nov. 22, 1990].
Where a pressure sensor is accommodated in a metal casing and the metal casing with the pressure sensor accommodated therein is joined and fixed on a base portion of a metal shell by seam welding as in Japanese Utility Model Application Laid-Open (Kokai) No. SHO 63-171988 or where a pressure sensor is sealed together with a molding material such as a resin within a metal casing and the metal casing is fixed via a flanged ring joined and fixed on a metal shell by seam welding as in Japanese Patent Publication (Kokoku) No. HEI 2-54637, it is only at an opening for a lead wire that the interior of the metal casing in which the pressure sensor composed of a piezoelectric element, etc. is accommodated is communicated with the outside. Accordingly, the watertightness of the metal casing with the pressure sensor accommodated therein is fully assured. The construction of the pressure sensor itself, however, becomes complex and the efficiency of production work is lowered, leading to the drawback that the above construction is very disadvantageous for the production of spark plugs produced in a large quantity.